Exhaust gas discharged from an internal combustion engine incorporated into a vehicle contains harmful chemical substances such as HC (hydrocarbon), CO (carbon monoxide) and NO.sub.x (nitrogen oxides). Therefore, conventionally, a catalyst converter used as an exhaust gas purifying device is arranged in an exhaust gas passage of the internal combustion engine. However, it is known that a rate of purification of a three-way catalyst used for the catalyst converter to purify the harmful substances contained in exhaust gas is low when the catalyst temperature is low, that is, when the catalyst is in an inactive condition. Accordingly, the conventional catalyst converter is disadvantageous in that exhaust gas can not be sufficiently purified when the catalyst is in an inactive condition immediately after the internal combustion engine has been started.
In order to solve the above problems, there is proposed an exhaust gas purifying device having an electrically heated catalyst device in which the catalyst support is made of metal and this metallic catalyst support is heated to a catalyst activity temperature (300 to 400.degree. C.) in a short period of time by supplying electricity when the internal combustion engine is started or the catalyst is in an inactive condition due to a low temperature.
As shown in FIG. 29, the exhaust gas purifying device is composed as follows. There is provided an external metallic cylinder 1 connected to the exhaust gas passage of an internal combustion engine. In the external metallic cylinder 1, there are provided an electrically heated catalyst support 2, which is heated by supplying electricity, and a main catalyst support 3, wherein the electrically heated catalyst support 2 and the main catalyst support 3 are arranged in series at a predetermined interval. In the electrically heated catalyst support 2, there is provided a central electrode 6, and an electrode section 8A of this central electrode 6 is led to the outside of the external metallic cylinder 1. Between this electrode section 8A and an external electrode 8B attached to a portion of the external metallic cylinder 1, there is arranged a battery 9 and a switch SW. When this switch SW is turned on, an electric current is fed to the electrically heated catalyst support 2 to be heated, so that heat is generated in the catalyst support 2.
As shown in FIG. 30(a), the electrically heated catalyst support 2 is composed as follows. A sheet of metallic foil 4 formed into a corrugated form, which will be referred to as a corrugated foil 4 hereinafter in this specification, and a flat sheet of metallic foil 5, which will be referred to as a flat foil 5 hereinafter in this specification, are put on each other, and the respective longitudinal end portions of the corrugated foil 4 and the flat foil 5 are joined and wound around the central electrode 6 while they are put on each other, so that a spirally-shaped metallic foil laminated body, which is referred to as a honeycomb body, can be formed. In general, these corrugated foil 4 and flat foil 5 are made of alloy of iron containing aluminum, for example, the composition is 20% Cr-5% Al-75% Fe, and the thickness of the foil is approximately 50 .mu.m.
FIG. 30(b) is a view showing one end face of the honeycomb body 7 composed of the metallic corrugated foil 4 and the flat foil 5. In honeycomb body 7, the corrugated foil 4 and the flat foil 5 are put on each other and wound round the central electrode 6. As a result, around the central electrode 6, there are formed a large number of spiral passages P in the axial direction formed by clearances between the corrugated foil 4 and the flat foil 5 of the honeycomb body 7. On the surfaces of the corrugated foil 4 and the flat foil 5, exhaust gas purifying catalysts are supported. Due to the above structure, when exhaust gas flows in the above passages P under the condition that the switch SW shown in FIG. 29 is turned on and the honeycomb body 7 is fed with an electric current so that the catalyst temperature is raised to a temperature of catalyst activity, the harmful components contained in exhaust gas come into contact with the catalyst, so that the exhaust gas can be purified.
In the exhaust gas purifying device composed in the above manner, by the influence of exhaust gas of high temperature flowing at high speed in the exhaust gas passage, and also by the influence of a heat cycle produced by the internal combustion engine which starts and stops, and also by the influence of vibration transmitted from the engine and the vehicle body to the catalyst, slippage is caused between the corrugated foil 4 and the flat foil 5 of the honeycomb body 7. This slippage will be referred to as "telescoping" in this specification, hereinafter. Further, the corrugated foil 4 and the flat foil 5 of the honeycomb body 7 are damaged. In this way, there is a possibility that the function of the electrically heated catalyst support is deteriorated.
In order to prevent the occurrence of telescoping between the corrugated foil 4 and the flat foil 5, and also to prevent the occurrence of damage of the corrugated foil 4 and the flat foil 5, for example, as shown in FIG. 32, British Patent Publication No. 1452982 discloses the following honeycomb body holding structure. An end face of the honeycomb body 7 on the downstream side of exhaust gas is supported by a cross-shaped metallic bar 70 which crosses an end face of the honeycomb body 7 in a contact condition, and an end portion of this metallic bar is fixed onto the external cylinder 1.
However, when the above honeycomb body holding structure is applied to an electrically heated metallic catalyst converter, the following problems may be encountered. When the honeycomb body 7 is energized with an electric current, the electric current bypasses the catalyst support and flows to the metallic bar 70, so that the heat generating efficiency of the honeycomb body 7 in the electric passage is greatly lowered and the function of the electrically heated catalyst support is deteriorated. Further, when a difference of temperature between the metallic bar 70 and the external metallic cylinder 1 is large, that is, when a difference of the thermal expansion coefficient between the metallic bar 70 and the external metallic cylinder 1 is large, the catalyst converter tends to be damaged.
On the other hand, Japanese Unexamined Patent Publication No. 5-269389 discloses an electrically heated catalyst device in which an end face of the honeycomb body on the downstream side is supported by one support bar covered with an electrically insulating strip, and this support bar is welded to an outside metallic housing (external cylinder).
However, this converter device is disadvantageous in that a welding portion between the support bar and the outside housing tends to be damaged by a difference of temperature and also by a difference of the thermal expansion coefficient. Further, there is a possibility of occurrence of telescoping of the honeycomb body in a portion not supported by the support bar.
In the above catalyst device, the following problems may be also encountered. In an exhaust gas purifying device in which a main metallic catalyst support is arranged on the downstream side of exhaust gas of the electrically heated catalyst, in order to enhance the warm-up property of the main catalyst support, the electrically heated catalyst support and the main catalyst support are located as closely as possible to each other so that heat can be effectively transmitted from the electrically heated catalyst support to the main catalyst support. In the above case, there is a possibility that the fixing pin to fix the support bar comes into contact with the main catalyst support, and the electrically heated catalyst is short-circuited, so that the heating performance of the electrically heated catalyst support is affected.
As described above, any conventional honeycomb body holding structure has a shortcoming. Therefore, the conventional honeycomb body holding structure is not appropriate for holding a honeycomb body of an electrically heated metallic catalyst carrier.
It is an object of the present invention to provide an electrically heated metallic catalyst in which the aforementioned problems of the conventional honeycomb body holding structure are not caused.
In other words, an object of the present invention is to provide an electrically heated catalyst device characterized in that: the occurrence of telescoping of the electrically heated catalyst support is prevented; the weight of the holding structure of the electrically heated catalyst support is not extremely increased; and there is no possibility of damage of the supporting bar and the supporting bar holding member, which tends to be caused by a difference of the thermal expansion coefficient in the operation of an engine after the electrically heated catalyst device has been incorporated into a vehicle, whereby the durability can be enhanced.
It is another object of the present invention to provide an exhaust gas purifying device in which the warm-up property can be enhanced by arranging the electrically heated catalyst support as closely as possible to the main catalyst support made of metal, without causing a short-circuit.